In the fabrication of metal oxide semiconductor (MOS) integrated circuits, it is customary to utilize field oxides surrounding active elements, or regions, to establish electrical isolation between each active region and its neighbors. The field oxide is formed before doping of active transistor regions and must be patterned. A process of local oxidation known as LOCOS is commonly used for formation of the field oxide. According to the LOCOS process, a layer of silicon nitride is deposited by chemical vapor deposition on the surface of a wafer. A photoresist is then applied over the silicon nitride and patterned in a photomasking operation. The photoresist is developed and rinsed to remove exposed portions, and exposed areas of the silicon nitride are removed in a plasma etch step. Next, the photoresist is stripped from the device and a thermal oxidation step is used to form an oxide layer in the range of 5,000-10,000 angstroms thick. As the oxide grows, additional oxide is formed by diffusion of steam through the gradually growing oxide layer.
The diffusion of steam is an isotropic process and undercuts the silicon nitride masking layer. The undercut portion is tapered away from the field oxide layer toward the active region and is commonly known as a "bird's beak" because of its shape. The silicon nitride is later removed from the surface of the device, leaving the field oxide with tapered bird's beak edges at the intersections with the active regions. The field oxide is left with a shallow angle taper, typically less than 45.degree., as it intersects the active region. The bird's beak typically has a lateral dimension of 0.5 to 1.0 micrometer. When elements of the semiconductor device have dimensions of the same order, the bird's beak can present significant problems.
In subsequent processing steps, it is necessary to deposit an oxide layer over the wafer to form contact holes, or vias, by conventional photomasking techniques, and to then deposit a metal layer to make contact with the active region of the transistor. The tapered bird's beak region around the periphery of the device creates a problem in establishing metal contact to the active region. If the contact photoresist hole lies over the edge of this tapered oxide, then the end of the bird's beak is etched away during the contact etch. Also, the thin field oxide layer adjacent the edge of the active region provides a relatively low resistance path between the metal contact and the underlying substrate. As a result, leakage or short circuits can occur around the junction formed by the active region and the underlying substrate.
In the past, this problem has been alleviated by enlarging the active region in relation to the contact region so as to provide sufficient overlap that leakage paths are avoided. The drawback of this technique is that some of the substrate area is occupied by the larger active region which does not contribute to device performance, but merely obviates creation of the leakage paths. When tens of thousands of these devices are thus constructed on a chip, there is a substantial impact on device density. Therefore, it is desirable to overcome the leakage problem without adversely affecting device size and density.
A recently developed technique for forming the field oxide in MOS devices is known as sealed interface local oxidation (SILO). The SILO process is described by, for example, J. Hui et al in "Electrical Properties of MOS Devices Made With SILO Technology," IEDM, May 1982, pp. 220-223. The process involves the formation of a nitride sealing film on the surface of the silicon, preventing lateral diffusion of the steam or other oxidizing species under the nitride masking layer. The SILO process for forming field oxide has provided nearly vertical steps at the interface between the field oxide layer and the active device.
It is a general object of the present invention to provide a novel method for semiconductor device fabrication.
It is a further object of the present invention to provide a method for semiconductor device fabrication wherein the bird's beak is eliminated as a leakage path between metal contacts and the underlying substrate.
It is a further object of the present invention to provide a method for semiconductor device fabrication wherein an oxide sidewall spacer is used to eliminate leakage paths between metal contacts and the underlying substrate.
It is a further object of the present invention to provide a method for semiconductor device fabrication wherein leakage from metal contacts is eliminated without enlarging the area to which contact is made.
It is a further object of the present invention to provide a semiconductor device wherein leakage paths between metal contacts and the underlying substrate are eliminated without enlarging the active region which is contacted.